Tools made of light – a celebration of laser physics

Arthur Ashkin is just four years shy of his 100th birthday. At the age of 96 the American physicist bagged the most coveted prize on earth for an invention he has been credited for past three decades. Arthur Ashkin, the oldest laureate of a Nobel in Physics ever, had developed the ‘optical tweezers’ technique back in 1986 that has led to an explosion of biophysical research. New York-born Arthur Ashkin invented optical tweezers that grab particles, atoms, viruses and other living cells with their laser beam fingers. This new tool allowed Ashkin to realize an old dream of science fiction – using the radiation pressure of light to move physical objects. He succeeded in getting laser light to push small particles towards the centre of the beam and to hold them there. Optical tweezers had been invented. A major breakthrough came in 1987, when Ashkin used the tweezers to capture living bacteria without harming them. He immediately began studying biological systems and optical tweezers are now widely used to investigate the machinery of life. In many laboratories, optical tweezers are today standard equipment for studying biological processes, such as individual proteins, molecular motors, DNA or the inner life of cells. Optical holography is among the most recent developments, in which thousands of tweezers can be used simultaneously, for example to separate healthy blood cells from infected ones, something that could be broadly applied in combatting malaria. Arthur Ashkin never ceases to be amazed over the development of his optical tweezers, a science fiction that is now our reality.

When the Royal Swedish Academy of Sciences announced on October 2 that this year’s Nobel Prize in Physics going to be shared by three scientists there were more surprises. The two other scientists – Gérard Mourou and Donna Strickland, who’re to share the accolade along with Ashkin – also worked for the advancement of laser physics. In a nutshell it’s a celebration of modern scientific advancements in laser physics. There is more reason to rejoice – Donna Strickland, a Canadian, became only the third woman recipient of a Nobel Prize in Physics after Marie Curie in 1903 and Maria Goeppert-Mayer in 1963. In between 1901 and 2018 as many as 112 received Nobel Prizes in Physics with John Bardeen receiving the Physics Prize twice but it took nearly half a century for another woman scientist to get one. “We need to celebrate women physicists because we’re out there,” said Strickland, a University of Waterloo professor, on Tuesday after being notified of the award, “and maybe in time it will move forward. I’m honored to be one of those women.”

Along with Strickland, French scientist Gérard Albert Mourou, a pioneer in the field of electrical engineering and lasers, co-invented a technique called chirped pulse amplification, or CPA, which was later used to create ultrashort-pulse, very high-intensity (terawatt – equal to one trillion watts) laser pulses. In the words of The Royal Swedish Academy of Sciences, “The inventions being honoured this year have revolutionised laser physics. Extremely small objects and incredibly rapid processes are now being seen in a new light. Advanced precision instruments are opening up unexplored areas of research and a multitude of industrial and medical applications.”

Gérard Mourou and Donna Strickland paved the way towards the shortest and most intense laser pulses ever created by mankind. Their revolutionary article was published in 1985 and was the foundation of Strickland’s doctoral thesis. Using an ingenious approach, they succeeded in creating ultrashort high-intensity laser pulses without destroying the amplifying material. First they stretched the laser pulses in time to reduce their peak power, then amplified them, and finally compressed them. If a pulse is compressed in time and becomes shorter, then more light is packed together in the same tiny space – the intensity of the pulse increases dramatically. Strickland and Mourou’s newly invented technique, called chirped pulse amplification, CPA, soon became standard for subsequent high-intensity lasers. Its uses include the millions of corrective eye surgeries that are conducted every year using the sharpest of laser beams.

Travelling in beams of light

Arthur Ashkin had a dream – a dream to put beams of light to work and made it to move objects. Immediately after the invention of the first laser in 1960, Ashkin began to experiment with the new instrument at Bell Laboratories outside New York. Ashkin thought that a laser would be the perfect tool for getting beams of light to move small particles. He illuminated micrometre-sized transparent spheres and, sure enough, he immediately got the spheres to move. At the same time, Ashkin was surprised by how the spheres were drawn towards the middle of the beam, where it was most intense. The explanation is that however sharp a laser beam is, its intensity declines from the centre out towards the sides. Therefore, the radiation pressure that the laser light exerts on the particles also varies, pressing them towards the middle of the beam, which holds the particles at its centre. To also hold the particles in the direction of the beam, Ashkin added a strong lens to focus the laser light. The particles were then drawn towards the point that had the greatest light intensity. A light trap was born; it came to be known as optical tweezers.

Living bacteria captured by light

After several years and many setbacks, individual atoms could also be caught in the trap. There were many difficulties: one was that stronger forces were needed for the optical tweezers to be able to grab the atoms, and another was the heat vibrations of the atoms. It was necessary to find a way of slowing down the atoms and packing them into an area smaller than the full-stop at the end of this sentence. Everything fell into place in 1986, when optical tweezers could be combined with other methods for stopping atoms and trapping them.

While slowing down atoms became an area of research in itself, Arthur Ashkin discovered an entirely new use for his optical tweezers – studies of biological systems. It was chance that led him there. In his attempts to capture ever smaller particles, he used samples of small mosaic viruses. After he happened to leave them open overnight, the samples were full of large particles that moved hither and thither. Using a microscope, he discovered these particles were bacteria that were not just swimming around freely – when they came close to the laser beam, they were caught in the light trap. However, his green laser beam killed the bacteria, so a weaker beam was necessary for them to survive. In invisible infrared light the bacteria stayed unharmed and were able to reproduce in the trap.

Accordingly, Ashkin’s studies then focused on numerous different bacteria, viruses and living cells. He even demonstrated that it was possible to reach into the cells without destroying the cell membrane.

Ashkin opened up a whole world of new applications with his optical tweezers. One important breakthrough was the ability to investigate the mechanical properties of molecular motors, large molecules that perform vital work inside cells. The first one to be mapped in detail using optical tweezers was a motor protein, kinesin, and its stepwise movement along microtubules, which are part of the cell’s skeleton.

New technology for ultrashort high-intensity beams

The second part of this year’s prize – the invention of ultrashort and super-strong laser pulses – also once belonged to researchers’ unrealised visions of the future. The inspiration came from a popular science article that described radar and its long radio waves. However, transferring this idea to the shorter optical light waves was difficult, both in theory and in practice. The breakthrough was described in the article that was published in December 1985 and was Donna Strickland’s first scientific publication. She had moved from Canada to the University of Rochester in the US, where she became attracted to laser physics by the green and red beams that lit up the laboratory like a Christmas tree and, not least, by the visions of her supervisor, Gérard Mourou.

One of these has now been realised – the idea of amplifying short laser pulses to unprecedented levels. Laser light is created through a chain reaction in which the particles of light, photons generate even more photons. These can be emitted in pulses. Ever since lasers were invented, 58 years ago, researchers have endeavoured to create more intense pulses. However, by the mid-1980s, the end of the road had been reached. For short pulses it was no longer practically possible to increase the intensity of the light without destroying the amplifying material.

Strickland and Mourou’s new technique, known as chirped pulse amplification, CPA, was both simple and elegant. Take a short laser pulse, stretch it in time, amplify it and squeeze it together again. When a pulse is stretched in time, its peak power is much lower so it can be hugely amplified without damaging the amplifier. The pulse is then compressed in time, which means that more light is packed together within a tiny area of space – and the intensity of the pulse then increases dramatically.

It took a few years for Strickland and Mourou to combine everything successfully. In 1985, Strickland and Mourou were able to prove for the first time that their elegant vision also worked in practice. The CPA-technique invented by them revolutionised laser physics. It became standard for all later high-intensity lasers and a gateway to entirely new areas and applications in physics, chemistry and medicine. The shortest and most intense laser pulses ever could now be created in the laboratory.

A laser’s extremely high intensity makes its light a tool for changing the properties of matter: electrical insulators can be converted to conductors, and ultra-sharp laser beams make it possible to cut or drill holes in various materials extremely precisely – even in living matter. For example, lasers can be used to create more efficient data storage, as the storage is not only built on the surface of the material, but also in tiny holes drilled deep into the storage medium. The technology is also used to manufacture surgical stents, micrometresized cylinders of stretched metal that widen and reinforce blood vessels, the urinary tract and other passageways inside the body.

There are innumerable areas of use, which have not yet been fully explored. Every step forward allows researchers to gain insights into new worlds, changing both basic research and practical applications. One of the new areas of research that has arisen in recent years is attosecond physics. Laser pulses shorter than a hundred attoseconds (one attosecond is a billionth of a billionth of a second) reveal the dramatic world of electrons. Electrons are the workhorses of chemistry; they are responsible for the optical and electrical properties of all matter and for chemical bonds. Now they are not only observable, but they can also be controlled.

(Acknowledgement: This writer substantially borrowed information and scientific explanations from the Nobel Prize’s official site)

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